1. Field of the Invention
This invention relates to wire enamels, and more particularly, to imide-modified polyester containing wire enamels, the imide-modified polyester containing both cycloaliphatic and aromatic imide-rings in the molecule, and materials used in preparing the imide-modified polyesters.
2. Description of the Prior Art
Wire enamels have been supplied heretofore in the form of a solution of a resin for the enamel dissolved in an appropriate solvent, with the solution having an appropriate viscosity suitable for wire forming operations. When wire enamels are diluted to a specific concentration, those wire enamels having better heat resistance generally have higher viscosity. Accordingly, when enamels are used in actual wire forming operations, they tend to be used in a lower concentration to achieve an optimum viscosity. On the other hand, the active ingredient in wire enamels is the resin material dissolved in the solution, and the solvent is used merely for dissolving the resin therein. The solvent is not directly effective at all for the final purpose of electrical insulation. In addition, wire enamels of lower concentration require a larger amount of energy for evaporation of the solvent in the baking thereof, and therefore, wire enamels of lower concentrations are not economically advantageous.
Presently, conservation of resources and energy is strongly desired, and effective utilization of natural resources and materials is an important concern. It is also important to prevent environmental pollution. As a result it is clear that a decrease in the amount of solvent used in wire enamels is extremely significant.
In general, various methods for reducing the viscosity of wire enamels are known, and some of the methods are as follows: (1) The concentration of the enamel resin can be reduced to reduce the viscosity of the enamel. (2) The enamel resin can be heated to reduce the viscosity of the enamel. (3) The molecular structure of the enamel resin can appropriately modified to reduce the viscosity of the enamel, for example, the polymerization degree of the resin can be reduced. Method (1) is a common method which has now been broadly adopted in this field, but this method is not desirable from the standpoint of economization of natural resources. Therefore, Method (1) is not related to this invention. Method (2) has also already been adopted in this field by heating the wire enamel to a certain temperature range. For example, it is well knwon to heat a wire enamel to 40.degree. to 60.degree.C for use in wire forming operations. However, a temperature of 40.degree. to 60.degree.C is not suitable for satisfactorily increasing the concentration of conventional wire enamels and yet achieving an appropriate viscosity range for wire forming operations. Thus, a further increase in the temperature in operation is inevitable. However, at these further elevated temperatures a hardener component in the enamel sometimes reacts due to the higher temperature or the solvent often evaporates due to the higher temperature, consequently resulting in an increase in the viscosity of the enamel after use for a long period of time. Some problems also exist in Method (3) where the degree of polymerization is reduced, or the molecular structure of the resin is modified to lower the melting point of the resin or to reduce the viscosity of a solution of the resin. In particular, special attention must be paid to changes so that the wire production operations, the appearance of the enamel wire and the heat resistance and mechanical properties of enamel wire are not degraded.